1. Field of the Invention
The present invention relates to a circuit board and processing method thereof. More particularly, the present invention relates to a circuit board and processing method thereof that can increase the wiring density of the circuit board.
2. Description of the Related Art
In today's information critical society, the market for electronic devices that facilitate information exchange expands at a tremendous pace. To increase the processing speed, provide more powerful functions, raise the level of integration and miniaturization and lower the selling price, chip packaging techniques are all aiming toward a higher degree of miniaturization and a higher degree of packaging density. In order to match this trend, the carrier in chip packaging, in particular, the ball grid array (BGA) and the pin grid array (PGA), has a higher integration level of wiring layout. Because a rigid substrate can provide a circuit layout with a higher wiring density and a higher pin count, it has become one of the most commonly used carriers in high-density chip packaging production. At present, the processes of fabricating a multi-layered rigid substrate having organic dielectric material layer can be categorized into the lamination method and the build-up method.
FIG. 1 is a schematic cross-sectional view of a conventional laminated circuit substrate. As shown in FIG. 1, the circuit substrate 100 mainly comprises a plurality of circuit layers 112, 114, 116, 118, a plurality of dielectric layers 122, 124, 125, two solder mask layers 132, 134 and a plurality of plated through holes (PTHs) 140. The circuit layers 112˜118 and the dielectric layers 122˜126 are alternately laminated over each other. The two solder mask layers 132, 134 are disposed on the topmost circuit layer 112 and the bottommost circuit layer 118 respectively to expose a portion of each of the circuit layers 112 and 118 so that they can connect electrically with a chip or other carrier in a subsequent operation. The plated through holes 140 pass through the circuit layers 112˜118 and the dielectric layers 122˜126, and electrically connect with the circuit layers 112˜118.
The conventional method of forming the plated through holes 140 includes performing a mechanical drilling process to form a plurality of through holes passing through various layers after the circuit layers 112˜118 and the dielectric layers 122˜126 are pressed together. Thereafter, the through holes are filled using a conductive material. Since the drilling density in all circuit layers 112˜118 is identical, the circuit wiring area between the top circuit layer 112 and the bottom circuit layer 118 is reduced. Thus, some of the circuits is forced to route through the inner circuit layers 114 and 116 and damage the integrity of the circuit layers 114 and 116 for serving as a ground plane and a power plane. In other words, the electrical performance of the circuit board 100 is compromised. Similarly, too high a drilling density in the inner circuit layer 114 and 116 will also lead to the aforementioned problem. In addition, routing the circuit lines is increasingly difficult when the density of drilled holes in the laminated circuit board 100 is too high. In some cases, even the costlier built-up type of circuit board or a laminated circuit board with more layers is required.